Project Summary/Abstract: Sex differences in immune responses are well-recognized, with females being less susceptible to infection, but 10X more likely to develop autoimmune disease than males. The involvement of individual sex-specific factors, including: sex hormones, sex chromosome complements and microbiomes, have been identified. However, there is a critical gap in our understanding of the mechanisms by which these factors mediate sex-specific responses. In addition, it is not known if or how these factors interact to regulate the overall responsiveness of the male vs. female immune systems. The current proposal will test whether gut microbiome metabolites, specifically short-chain fatty acids (SCFAs), induce sex-specific responses by promoting the bi-allelic expression of X-linked immune-related genes in females. Most previous microbiome studies have focused on identifying sex-specific microbiome populations with distinct metabolic capabilities to explain sexually dimorphic responses. However, our preliminary data suggests that similar levels of SCFAs can differentially regulate humoral immune responses in a manner that is XX-dependent. We hypothesize that SCFAs contribute to sexually dimorphic immune responses by utilizing epigenetic regulatory mechanisms to differentially influence immune-related gene expression in an XX vs. XY- dependent manner. This hypotheses will be addressed in two Specific Aims. Specific Aim 1 will determine if SCFA-mediated HDAC inhibition increases B cell activation by promoting the bi-allelic expression of X-linked immune-related genes. Specific Aim 2 will evaluate the potential for SCFAs to enhance humoral immune responses to heat-killed Streptococcus pneumoniae (HKSP)-immunization in an XX-dependent manner. By defining the underlying mechanisms that contribute to sex-specific response, the proposed research will contribute to the development of better treatment strategies to combat sex-biased autoimmune and inflammatory diseases, and improved vaccine efficacies. These studies are also relevant as gut microbiome metabolites are being marketed as potential immuno-therapeutics, due to their immunomodulatory activities. Prebiotics, probiotics and SCFA supplements are readily available over-the-counter, and most recently, SCFAs have been proposed as treatments for several immune-related disorders. However, few studies have investigated the potential for these compounds to exert sex-specific immune effects and, to our knowledge, no previous study has addressed the possibility that such effects may occur in a sex chromosome complement-dependent manner.